There are three types of anesthesia commonly utilized to perform surgical procedures including local, regional and general. Local anesthesia is administered via local drug injections directly into the effective tissues, while regional anesthesia is accomplished by injecting anesthetics into the tissues adjacent to the nerve supply of a specific region, thus anesthetizing the area of surgery. General anesthesia depresses the central nervous system and is preferred for major operations. It is often induced with intravenous anesthetics such as non-volatile drugs, and maintained with inhalation (volatile) anesthetics. The most commonly used anesthetic gas is nitrous oxide, in combination with oxygen and an intravenous or volatile anesthetic such as isoflurane, halothane, enflurane, desflurane and sevoflurane. A 100% concentration of nitrous oxide is lethal to a patient, hence the need for intermixture with oxygen, and a volatile anesthetic is necessary because of a comparatively low potency of nitrous oxide.
The delivery of inhalation anesthetics to patients has been the subject of a great deal of development since surgical anesthetics were first introduced in 1846. Inhalation anesthetics are delivered to the patient by ventilation, which is a process distinct from anesthesia but acts as a convenient delivery medium. Ventilation itself is the process of bringing oxygen into the alveoli of the lungs, and washing out carbon dioxide from the lungs. Three types of ventilation are possible during inhalation anesthesia, including spontaneous, assisted or controlled. In spontaneous ventilation, the patient breathes at his or her own pace without any assistance from the anesthesiologist or ventilation machine, although some ventilation machines are capable of assisting the patient to reduce his or her work of breathing. Assisted ventilation, also known as manual ventilation or "handbagging," is performed by the anesthesiologist manually squeezing a breathing bag attached to the anesthesia breathing circuit to supplement the spontaneous ventilation of the patient.
During general anesthesia, where the patient is essentially paralyzed by neuromuscular blocking agents and the anesthetic gas, mechanically controlled ventilation is utilized to automatically ventilate the patient's lungs. Various modes of mechanical or automatic ventilation are currently used including closed-circuit, open-circuit, high frequency ventilation and continuous-flow apneic ventilation, among others. High frequency ventilation is characterized by the introduction of pulsed or intermittent, relatively small volumes of air into the patient's lungs. This mode of ventilation is especially useful in extracorporeal shock wave lithotripsy which is a non-invasive way of pulverizing kidney stones with focused shock wave energy. Continuous-flow apneic ventilation introduces high flow rates of gas via catheters inserted in the patient's bronchi, and relies on the continuous flow of gas to bring in oxygen and wash out carbon dioxide. Closed and open circuit ventilation are generally differentiated by whether or not the gas flow is completely recirculated to the patient (closed systems), or never recirculated for re-inhalation by the patient (open system) and variations in between (semi-closed, semi-open).
Although a number of inhalation anesthesia systems have been proposed, the most common commercial design employed in this country and much of the world consists of four main components, including: (1) an anesthesia machine, (2) an anesthesia ventilator, (3) an anesthesia circuit, and, (4) a scavenging system. The "anesthesia machine" is an assembly for the blending of air, helium and/or oxygen with nitrous oxide and one or more volatile anesthetics such as isoflurane, halothane, enflurane, desflurane and sevoflurane. The air, oxygen and nitrous oxide are intermixed within a common manifold containing rotameters which provide independent metering of each gas. The intermixed gases exit the manifold and are directed through a vaporizer, usually of the flow-over type, where the volatile anesthetic is added.
The gas mixture produced by the anesthesia machine enters the "anesthesia circuit" which is connected to the patient. A number of anesthesia circuits can be utilized, but many commercial designs employ a "circle" system for delivery of the gas mixture to a patient via a facemask fitted over the nose and mouth of a patient, or an endotracheal tube with which the patient is intubated. A circle anesthesia circuit includes a circulation loop having an inlet connected to the anesthesia machine, an outlet connected to the scavenging system and a Y-piece or other means of connection to the patient. The circulation loop carries an inspiratory valve which is a one-way valve allowing passage of the gas mixture to the patient, and an expiratory valve which is a one-way valve permitting passage of the gas exhaled by the patient. If the circle anesthesia circuit is operated in an "open" mode, gas exhaled from the patient is transmitted directly to a scavenging system for collection and disposal. In the closed or semi-closed mode of operation, all or part of the gas exhaled by the patient is recirculated within the circulation loop, and a carbon dioxide absorber is mounted therein to remove carbon dioxide from the gas flow before it is directed back to the patient.
Circle anesthesia circuits are normally equipped with a manual breathing bag which allows the anesthesiologist to manually ventilate the patient by squeezing the bag. Manual ventilation requires constant attention on the part of the anesthesiologist, and ties up at least one hand. To free the clinician's hand, once the anesthetic is safely under way, the anesthesia ventilator is connected to the circulation loop to provide for mechanical ventilation of the patient. In switching from manual to mechanical ventilation, a valve (e.g., a selector knob) in the system is manually operated to block the passage to the manual breathing bag, and interconnect the anesthesia ventilator with the circulation loop. Typically, an ascending bellows ventilator is incorporated in the anesthesia ventilator, which, under pressurization from gas supplied by the ventilator, is operative to alternately descend to inflate and ventilate the patient's lungs, and ascend passively from the build-up of exhaled gases from the patient. A ventilator exhaust valve is also provided to prevent over-pressurization. During exhalation, after the bellows has fully ascended, the ventilator exhaust valve opens and allows the excess gases to spill into the scavenging system, so that there is no net buildup of pressure.
Efforts have been made to improve the operation of anesthesia delivery systems of the type described above, particularly in connection with incorporating electronic control features and data acquisition sensors. For example, sensors have been employed at the connection of the circuit to the patient for the determination of the inspired and exhaled gas concentrations during an anesthetic procedure. A feedback loop can be employed to control the introduction of new anesthetic into the breathing circuit dependent upon the concentration of anesthetic sensed by the sensors. Anesthesia delivery systems provide for "flushing" of the circuit by a high flow rate of oxygen in order to remove an undesirable concentration of carbon dioxide and/or anesthetics which may be present, or to compensate for loss of circuit volume following, e.g., a momentary disconnection.
Despite advances in anesthesia delivery systems, a number of problems remain which limit their efficiency, economic benefit, and, in some instances, impact adversely upon patients' safety. One pervasive problem with existing anesthesia delivery systems is that they deliver expensive anesthetic gases inefficiently and at flow rates much higher than necessary. In a high percentage of instances, less than 5% of the gas output from currently used anesthesia machines is taken up by the patient, and the remaining 95% is spilled into hospital scavenging systems or into the ambient environment of the operating room. Based on recent studies, concerns have arisen over the possible link between continued exposure to waste anesthetic gases and higher than average rates of spontaneous miscarriage, birth defects, and cancer among operating room personnel. Further, anesthetic gases collected by hospital scavenging systems are ultimately vented to the atmosphere. Nitrous oxide, which has an estimated medical use of 100,000 tons per year worldwide, has been found to deplete the atmosphere's ozone layer. Volatile anesthetics such as isoflurane, halothane and enflurane are chlorofluorocarbons which contribute to both ozone layer depletion and the greenhouse effect. The amount of chlorofluorocarbons released into the atmosphere from volatile anesthetics is estimated to be in excess of 2,000 tons per year. In addition to the environmental and health hazards of anesthetic release into the atmosphere, the economic loss can be substantial. Even assuming a high rate of anesthetic usage efficiency of about 50%, it is estimated that about $225,000,000 per year is wasted world wide on the loss of volatile anesthetics alone, excluding nitrous oxide and oxygen. This estimate may increase substantially if more expensive, improved volatile anesthetics like desflurane and sevoflurane become widely used.
In addition to the foregoing, commercially available anesthesia delivery systems have a number of operational limitations which detract from their usefulness and safety. In most widely used commercial designs, mechanical ventilation of the patient is possible only when the system is connected to a continuous supply of gas, e.g. from the hospital gas supply system. It is not possible to mechanically ventilate a patient with ambient, room air with these types of systems. Further, the ventilators of most anesthesia delivery systems cannot operate without a supply of compressed gas from cylinders or the like. As noted above, even though sensor arrays and electronic feedback loops have been incorporated to some degree in current anesthesia ventilator systems, the data acquired by such sensors often fail to accurately portray the actual parameters to be sensed, i.e. tidal volume, flow rate, pressure and the like. Incorrect or inaccurate measurements or data can have serious adverse effects, such as the gradual augmentation of the desired tidal volume supplied to the patient during mechanical inspiration.